The Newsletter of Pheromone-based Orchard Pest ManagementVol. 1, No. 5 -- May 15, 1996
CAMP takes on the codling Mothra!
John E. Dunley
Washington State University Tree Fruit Research and Extension Center
Pesticide resistance is a problem which has been faced by virtually every apple and pear grower. Two orchard pests, spider mites and pear psylla, are particularly renowned for their abilities to develop resistance to nearly every pesticide used to control them. Codling moth, the key pest of pome fruits, also has an impressive (but often overlooked) record of developing resistance rapidly. Starting in the 1930's, problems with lead arsenate resistance in codling moth threatened the fruit industry. Fortunately, DDT was introduced in 1947 and saved the crops. DDT worked very well for the first 5 to 10 years, then resistance became widespread by the late 1950's. Just when codling moth seemed destined to rule the orchards once again, the organophosphate insecticides were introduced. One OP in particular, azinphosmethyl (Guthion), had better residual control than others such as diazinon, and Guthion became the standard for codling moth control. Once again codling moth was manageable.
Guthion has been used for over thirty-five years in the western US for control of codling moth. This long-term use of Guthion has been described as one of the factors leading to successful IPM implementation in tree fruits. Because Guthion has remained effective for so long, several key natural enemies have been able to evolve resistance. Thus, some biological control occurs even where Guthion is used (e.g., biological control of spider mites). However, the recent re-emergence of resistance has been important to further IPM implementation efforts, as resistance management using pheromone mating disruption changes the direction of tree fruit pest management programs.
Guthion resistance was first found in codling moth in 1989, when it was documented from pear orchards in the Sacramento Delta of California by Drs. Lucia Varela and Steve Welter of the University of California at Berkeley. Subsequent surveys throughout the western US have found areas of resistance in Washington, Oregon, and Utah. Researchers in South Africa, Europe and Australia have also recently documented Guthion resistance, demonstrating that Guthion resistance in codling moth has become a global problem.
The development of resistance has already resulted in increased frequency of Guthion applications and rates per application in California and Washington. Many pear growers in the Sacramento delta region of California are applying maximum rates of Guthion each season to achieve satisfactory control. Because preliminary data suggests that resistance may be evolving at both regional and local levels, efforts to manage resistance are most appropriate on an areawide basis. Fortunately, the need to regionally manage resistance has coincided with areawide management efforts for codling moth using pheromone mating disruption, i.e., CAMP (Codling Moth Areawide Management Program).
Growers in areas with Guthion resistance experience greater difficulty controlling codling moth, even though resistance is still at levels which do not confer complete field failure. Relative to most other cases of resistance, Guthion resistance is problematic even when levels are quite low. Control problems usually begin when resistance is only 3- or 4-fold (this is the ratio of the amount of pesticide which kills half the test population to the amount necessary to kill half a susceptible population). The highest level of resistance found is only 12-fold. Resistance typically appears in the field as increased fruit infestation, and more frequent applications of maximum rates of Guthion are necessary to keep the damage in check. When resistance is present growers typically do not see much drop in pheromone trap catch following cover sprays. Trap catch in susceptible areas usually decreases quickly following covers.
While the exact mechanisms of Guthion resistance are not known at this time, the ability to detoxify Guthion also confers detoxification of a wide range of other pesticides (cross-resistance). While at UC-Berkeley I worked with Dr. Steve Welter to compare resistant and susceptible codling moth against several classes of insecticides in laboratory and field studies. Our work demonstrated a wide range of insecticides with cross-resistance, including other organophosphates (Diazinon and Imidan), pyrethroids (Asana and Danitol), carbamates (Sevin), chlorinated hydrocarbons (DDT), and insect growth regulators (Dimilin and Comply). Probably the most interesting and unexpected finding was the cross-resistance with insect growth regulators (IGRs). As IGRs are much more selective than the broad-spectrum neurotoxins used in the past, they are viewed as the next generation of chemical controls for tree fruit IPM programs. However, reducing the current levels of Guthion resistance may be necessary if IGRs are to be used effectively in the future. Thus, establishing Guthion resistance management programs not only will extend the usefulness of Guthion, but will allow softer IGR-based programs in the future. Reversion (reduction or reversal) of Guthion resistance may be critical to the successful use of combination programs of pheromone mating disruption and IGRs.
Another interesting finding in the cross-resistance studies was that two organophosphates, Penncap-M and Lorsban, were both negatively cross-resistant. This means that these compounds have greater effect on resistant moths than susceptible moths. If used correctly, these compounds can give chemical control of codling moth while reducing the levels of Guthion resistance, by killing a greater proportion of resistant moths than susceptible ones. Dr. Welter is currently investigating the use of Penncap for resistance management in the Randall Island Areawide Project, where codling moth pressure is very high and Guthion resistance is also high. However, these chemicals are not an optimal alternative as both are disruptive, and if used indiscriminately can greatly reduce the benefits of using soft programs and mating disruption. These chemicals should be viewed as a last resort, only to be used in situations where resistance has become a severe problem.
Use of pheromone mating disruption for codling moth control provides a non-chemical alternative to Guthion. Reducing the selection from Guthion allows reversion to occur, and reversion of Guthion resistance in codling moth should be attainable in the field. Laboratory studies in California and Washington have demonstrated that Guthion resistance is instable when further treatment with Guthion ceases. Complete reversion has occurred in resistant laboratory populations in California after 6-7 generations; in other words, a 6-fold resistant population became completely susceptible in only 7 generations. If CAMP continues to be successful in reducing Guthion use, a likely outcome will be reduction in levels of Guthion resistance.
The establishment of the areawide pilot projects for the management of codling moth using pheromone mating disruption provides a unique opportunity to practice proactive regional resistance management, before Guthion is lost because of high resistance levels. Additionally, the use of alternative tactics for codling moth control enables growers to use more diverse strategies, such as biological control of other pests. By developing practical resistance management programs for tree fruits now, with CAMP as an example, we will better manage the few effective compounds available, and move toward long-term stability in tree fruit IPM.
I interviewed nine orchard consultants with extensive experience monitoring for leafrollers, both pandemis (PLR) and obliquebanded (OBLR), in Washington state. All monitor intensively for orchard pests through the growing season, and use economic or action thresholds extensively in making control recommendations. Most are frustrated by the difficulty of finding leafroller larvae before damage is done, unless they spend far more time than they can afford searching for this pest. Larvae are quite small and well hidden, and distributed very unevenly in orchard blocks. Pheromone traps are of little value in monitoring leafroller populations, as they attract males from an extensive area that may not accurately represent the population level of that particular block. Consequently, many consultants rely almost on a "presence or absence" sampling scheme early in the season. "Gut feel" and intuition come into play, too. "The most inefficient, time-consuming and doubtful monitoring I do!" says one consultant.
Nonetheless, the interviews with the consultants, and leafroller researchers, do show considerable agreement on how to monitor for leafrollers as the season progresses. The key features include:
Block history of leafrollers: Find out if they were present last year, and where. Fruit checks from last harvest are valuable. Check packout records. Bear in mind that some warehouses may have leafroller fruit damage included with "worms" or misidentified as codling moth.
Early season sampling: Overwintering larvae emerge at or soon after green tip, but are very difficult to find. A Lorsban/oil application during delayed dormant is a given, particularly with any leafroller history. "Don't take chances with the overwintering generation," said one consultant. The larvae, and their damage, become dramatically easier to locate by pink. Most consultants began sampling at tight cluster or later to determine the need for Bt applications starting at pink.
Examine many trees: Leafrollers are not distributed uniformly across orchard blocks. Several consultants vary their monitoring route each time to improve their chance of finding hot spots. Examine at least 20 trees per 10-acre block. Monitor frequently in the critical period of pink through petal fall.
Look high in the tree: PLR larvae are concentrated high in the tree canopy; as many as 10 times more are found in the upper half of the canopy as in the lower half. Concentrate your search in the interior of the tree and, in spring, on clusters and shoots arising from big wood. In trees taller than 10 feet some means of searching the upper canopy (ladder, standing on a 4-wheeler, climbing trees) must be used to accurately detect PLR. OBLR larvae are more uniformly distributed within the tree canopy; sampling from the ground is probably accurate enough.
Sample at the right times: Determine the need for pre- and post-bloom treatments with samples of blossom clusters from tight cluster through petal fall. Leafroller larvae move to shoot tips, particularly on bourse shoots, beginning around bloom. Assess the larval population again about 2 weeks after petal fall, before they begin to pupate. This sample provides information about the control achieved with spring treatments. It also is the best measure of the need for summer controls, as it is almost impossible to accurately sample for the young larvae in late June and July before damage occurs. Look at shoot tips in the top half of the tree, examining 10-20 shoots each on at least 25 trees per 10-acre block. Many consultants will cut down and open up any damaged shoots, looking for leafroller presence, and determine from this the damage potential for the next generation.
Sampling summer generation larvae is difficult until they are big enough to roll leaves of growing terminals (mid to late July). By this time, most fruit damage by PLR larvae has already occurred. Some consultants spend time looking for small larvae, before leaves are rolled, along the undersides of leaves, or examine fruit that touches leaves, limbs or other fruit. Generally, this is a time-consuming sample of questionable accuracy: "I sample until my gut unclenches," says one consultant. Sampling terminals for larger larvae in July and August may have some value. If populations are high enough, treatment may be needed to prevent further fruit damage (particularly with OBLR) or to reduce the population that could threaten fruit at harvest. Again, sample shoots on many trees, at least 25 trees per 10 acres.
The action threshold (the population level at which treatment is recommended) used by these consultants is a bit vague, reflecting the difficulty of monitoring leafrollers accurately. I am cooperating with Jay Brunner and Larry Gut this year in field trials designed to assess the intensity and time needed for accurate larval monitoring through the season, and to relate population levels to fruit damage at harvest. When the consultants use a threshold, it tends to be as follows:
Tight cluster: mere presence to < 1% infested clusters.
Pink through petal fall: 1-2% infested clusters (a higher level than at tight cluster, as can more accurately monitor the population now).
Petal fall until end of overwintering generation (about late May): 1-2% infested shoots.
Summer generation: 1-2% infested shoots.
The threshold is lower with larger trees, in blocks with a history of extensive leafroller damage, with higher value cultivars, or with an orchard history of poor spray application or timing.
Carpenter Hill (near Medford, OR) - July 11, 1996
West Parker Heights (Yakima Valley, WA) - July 16, 1996
Howard Flat (near Chelan, WA) - July 18, 1996
Lake Osoyoos (near Oroville, WA) - July 25, 1996
AREAWIDE IPM UPDATE
The Newsletter of Pheromone-based Orchard Pest Management
Ted Alway, Editor
Phone: (509) 664-5540
Fax: (509) 664-5561
Partial Funding provided by: Washington State Tree Fruit Research Commission, U.S. Department of Agriculture-Agricultural Research Service,International Apple Institute, and U.S. Environmental Protection Agency
WSU Cooperative Extension, Chelan County
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Wenatchee, WA 98801